1. Field of the Invention
The present invention relates generally to extrusion blow molding machines and, more particularly, to the configuration of the barrel and ram of an accumulator head in an extrusion blow molding machine.
2. Description of the Related Art
Containers and other hollow articles are frequently formed by blow molding. The blow molding process involves providing a tube of heated and softened plastics material (parison) about which a two-piece mold is closed. Air or another gas is introduced under pressure into the parison to expand it against the walls of the mold, forming the desired hollow article. Such blow molding machines can be of various types, the most common of which are extrusion-blow molding machines and injection-blow molding machines.
In extrusion-blow molding machines, plastics material is heated and softened in an extruder and is conveyed into a die head from which a tubular parison is extruded. The parison can either be continuous, whereby a plurality of molds are sequentially enclosed about successive, axially spaced portions of the parison, or individual parisons can be intermittently extruded and subsequently blown. In the latter instance, the die head includes a chamber, usually called an accumulator, in which the extruded material is accumulated to provide a predetermined volume of material. The accumulated volume of material is then ejected through an annular die at the outlet of the accumulator to form a parison having the desired length, diameter, and wall thickness distribution.
Typically, an accumulator has a purging system to accomplish the function of ejecting the accumulated material by forcing it through the die at the outlet, and a programming system to control the die opening at the outlet for varying the wall thickness of the extruded parison and to close the die while plastic for the next parison is being fed into the accumulator. The speed at which material is ejected by the purging system is often synchronized with the size of the die opening as controlled by the programming system. This synchronization enables the wall thickness to vary in a controlled manner over the length of the parison.
During production operation, an accumulator head is designed to receive from the extruder a quantity of plastic melt equal to the size of the parison to be formed. To facilitate use in range of applications, the charge of plastic melt can be varied as desired, up to a maximum, which corresponds to the full volume of the accumulator chamber. Typically, accumulator heads are identified by their maximum capacity, for example, a "60 lb." head. In order to provide a design that will perform satisfactorily, the operating systems of the accumulator head must be sized to operate at the maximum capacity. From a practical standpoint, the accumulator operates most effectively over a relatively limited range of parison sizes, with the maximum capacity being the upper limit of that range. More specifically, there is better processing control if the dimensions of the accumulator chamber and the length of the ram stroke approximate the desired parison in diameter and length.
These factors governing accumulator design dictate that the manufacturer have several different accumulator constructions available to meet the market requirements for different size containers. As the market expands to new applications requiring larger and larger parisons, new accumulator heads must be designed to provide the desired capacity. The prior art has not provided an accumulator construction that can be easily modified to alter the capacity of the accumulator chamber.
Furthermore, in order for an accumulator to be economical to manufacture, it is desirable for accumulators of different capacities to have as many common parts as practical. However, the advantage of lower parts inventory must be balanced against compromises in the overall design that affect performance. Accumulator barrel constructions of the prior art require that the entire barrel be constructed for a high grade of steel to resist the wear of repeated operation of the purging system.